The present invention relates to a process for the concentration of polyunsaturated fatty acids present in fish oils.
Fish oils contain a complex mixture of fatty acid moieties, mostly straight chain with even numbers of carbon atoms. These moieties, mainly present as their glycerides, are either saturated or mono- or polyunsaturated. Whereas vegetable oils and fats from terrestrial animals are composed mainly of fatty acids containing a maximum of 18 carbons and two or three double bonds, fish and marine mammal oils are composed of substantial amounts of fatty acid moieties containing 20 or 22 carbons and four, five or six double bonds, Stansby, "Fish Oils" Avi Publishing Company, Inc. (1967). Among the fatty acid moieties unique to fish oils are the following n-3 compounds: 18:4, 20:4, 20:5, 22:4, 22:5, 22:6. As generally used, n-3 or omega-3 means that the first double bond begins at the third carbon counting from the methyl end of the chain. In the number:number designation, the first number indicates chain length and the second number indicates how many double bonds are present. For example, 18:4 indicates a straight chain comprised of 18 carbon atoms containing 4 methylene-interrupted double bonds.
The polyunsaturated components of fish and marine mammal oils are of particular interest because they differ significantly from those found in vegetable oils or fats from terrestrial animals. Highly unsaturated fatty acids, such as those contained in fish oils, are important dietary factors beneficial in reducing the development of atherosclerotic lesions, Dyerberg et al, "Nutritional Evaluation of Long-chain Fatty Acids in Fish Oil", pp 245-261, Academic Press, London (1982). Accordingly, there is great interest in obtaining the polyunsaturated fatty acids present in fish oils in a more concentrated form.
The separation of fatty acids is complicated by several factors. First, due to their high molecular weights, the differences in their molecular weights are relatively small making it difficult to separate them by conventional means. This is particularly the case when saturated and unsaturated fatty acids of the same chain length are to be separated. Second, polyunsaturated compounds are readily susceptible to polymerization, degradation and/or oxidation, even at moderately elevated temperatures.
Fatty acid moieties are most conveniently separated as their lower alkyl esters. Current methods for fractionating fish oil esters use urea complexing, which selectively removes mono-unsaturates such as 20:1 and 22:1, and/or adsorption and/or absorption chromatography, and/or fractional and/or molecular distillation, processes which are cumbersome and time consuming. Particularly undesirable are methods which require the use of difficult-to-remove organic solvents, and thus leave toxic residues in the fractionated esters. Methods involving heating at temperatures approaching 200.degree. C. introduce the possibility of alteration and the formation of toxic substances.
A recently reported method for separating various fatty acid components in cod fish oil uses a combination of extraction with supercritical carbon dioxide and fractionation, Eisenbach, Ber. Bunsenges. Phys. Chem., 88 882-887 (1984). The fractionation column contains stainless steel packings to provide an opportunity for equilibration between the individual fish oil esters and carbon dioxide. Fractionation is enhanced by a hot finger fitted into the top of the fractionation column. Heat reduces the density of the carbon dioxide/fish oil ester solution, and concommitantly decreases the solubility of all solutes, but not to the same extent. The more soluble components are removed and the less soluble components are returned and concentrate in the reactor vessel. Those less soluble components can be recovered from the reactor or alternatively can be extracted and removed through the fractionation column by reducing the temperature of the hot finger. Such a process, which involves fractionation, is rather time consuming and exposes the fatty materials being separated to thermal degradation.
An article in the December 1983-January 1984 issue of the Pacific Seafood Chronicle, West Coast Fisheries Development Foundation, discusses extraction of various materials with supercritical carbon dioxide. The article indicates, without giving details, that such extraction might also be used to separate highly unsaturated fatty acids from the more saturated acids present in fish oils.